I. Field of the Invention
The present invention relates generally to alignment systems for enabling an operator to position an ophthalmic instrument relative to an eye of a patient, and more particularly to an alignment system that is well-suited for use in a hand-held ophthalmic instrument and that provides an operator with a direct view of a patient""s eye as a positioning aid.
II. Description of the Related Art
Alignment systems for use by an operator in locating an ophthalmic instrument relative to an eye of a patient vary in complexity. In instruments where alignment is critical to measurement accuracy, for example in non-contact tonometers, it is commonplace to provide means for projecting a visible fixation target image along a measurement axis of the instrument to direct the patient""s gaze, and to further provide an opto-electronic position detection system capable of sensing the position of the instrument relative to the eye. Where the ophthalmic instrument is a non-contact tonometer having a discharge tube for directing a fluid pulse at the eye, X-Y alignment is typically achieved by aligning an axis of the discharge tube to intersect with the corneal vertex, and Z alignment is achieved by positioning a fluid exit end of the discharge tube at a predetermined distance from the corneal vertex.
U.S. Pat. No. 3,756,073 to Lavallee et al. describes a non-contact tonometer having a target projecting system that projects an image of a target along an alignment axis through an objective lens to the image plane of the objective lens. Consequently, when the image plane of the objective lens is coincident with the center of curvature of the patient""s cornea, a corneal virtual or mirror image of the target is re-imaged by the lens and a telescope lens in the plane of a circle reticle on the alignment axis. An operator looking through an eyepiece along the alignment axis toward the eye can see the retro-reflected target image superimposed on the circle reticle, and aligns the instrument laterally and vertically (X-Y alignment) by centering the target image with respect to the reticle markings. According to this system, the corneal surface under observation is limited to a desired small portion of the entire corneal surface. The ""073 patent also describes a passive xe2x80x9cgo/no goxe2x80x9d alignment confirmation system comprising an infra-red LED cooperating with an alignment detector located behind a pinhole aperture, whereby the detector generates a trigger signal upon alignment.
A more sophisticated opto-electronic alignment system for use in locating an ophthalmic instrument relative to an eye is taught in U.S. Pat. No. 4,881,807 to Luce et al. According to this system, and other systems of the prior art, triangulation is used to gauge the three-dimensional location of the eye relative to the instrument. By way of example, the aforementioned U.S. Pat. No. 4,881,807 discloses a system wherein two light sources arranged on opposite sides of the eye illuminate the eye with divergent rays, and a pair of CCD area detectors each comprising a two-dimensional array of light-sensitive pixels are arranged behind associated pinhole apertures to receive a small bundle of reflected rays originating from a corresponding one of the light sources. A local x-y location where the light strikes the CCD array is determined by identifying the pixel registering the peak response signal. The local x-y locations where light strikes each CCD array and specifications describing the predetermined geometric arrangement of the system components are provided as inputs to a microprocessor, which then calculates the amount of movement in the global X, Y, and Z directions necessary to achieve alignment. A video image detector is also provided to supply a macro-image of the eye to a CRT display, and output from the alignment CCD electronics is coupled into the CRT display electronics to provide alignment illumination spot symbols on the video display image.
Known alignment systems do not afford the operator a direct view of the eye along an alignment axis or main optical axis of the instrument for alignment purposes. In fact, many prior art systems rely on generating and displaying a video image of the eye and superimposing alignment cues in the displayed video image for moving the instrument to achieve alignment. This approach requires instrumentation that adds to the size, weight, and expense of the instrument.
Therefore, it is an object of the present invention to provide an alignment system for an ophthalmic instrument that affords the operator a direct view of the patient""s eye along an optical axis of the instrument.
It is another object of the present invention to provide an alignment system for an ophthalmic instrument that affords the operator a direct view of the patient""s eye along an optical axis of the instrument while simultaneously presenting a fixation target to the patient along the optical axis.
It is another object of the present invention to provide an alignment system for an ophthalmic instrument that includes an instructive display image superimposed with the directly viewed real image of the eye in the operator""s field of view.
It is yet another object of the present invention to provide an alignment system for an ophthalmic instrument that is relatively inexpensive to manufacture.
In furtherance of these and other objects, an ophthalmic instrument having a central optical axis to be aligned with a patient""s corneal vertex comprises an eyepiece along the optical axis for enabling an operator to directly view the patient""s eye for generalized alignment, and means for presenting a dark fixation target surrounded by a bright background to the patient along the same optical axis, wherein the bright background helps to illuminate the eye for operator viewing.
An alignment system according to a preferred embodiment further comprises an afocal position detection system for determining X-Y-Z alignment status of the instrument relative to the patient""s eye. The position detection system comprises first and second light sources on opposite sides of the central optical axis of the instrument, and corresponding first and second light-sensitive area detectors positioned to receive light from an associated light source after it has been reflected by the cornea. The detectors provide signal information indicative of the local x-y position of an illumination spot formed thereon. In a preferred embodiment, the first and second detectors are quad-cell detectors having four quadrants, and the illumination spot size is about the size of one quadrant, whereby the x-y position can be determined based on the four signal levels generated by the quadrants. Collector lenses after each light source and in front of each detector minimize vergence in the light beam as it illuminates the eye and as it arrives at a detector.
The local x-y data from each detector are then provided as input to a series of stored geometrical relationships determined during instrument calibration for giving the X-Y-Z global alignment status of the instrument relative to the eye. The geometrical relationships are multiple regression equations for X, Y, and Z, wherein regression coefficients for each equation are determined by reading local x-y data from the detectors for an artificial eye placed at a plurality of known X-Y-Z positions during calibration. The regression coefficients are stored during calibration and used during normal instrument operation to quickly calculate X, Y and Z coordinates based on local x-y data from the detectors as an operator positions the instrument relative to a patient""s eye.
A xe2x80x9cheads-upxe2x80x9d display is preferably connected to receive the X-Y-Z position data and provide instructional cues to the operator for moving the instrument to achieve alignment. In a current embodiment, the heads-up display comprises a polar array of light emitting diodes selectively illuminated to indicate a desired X-Y movement direction, and a linear array of light emitting diodes selectively illuminated to indicate a desired Z movement direction. An image of the heads-up display is presented to the operator along the instrument optical axis through the use of a beamsplitter that allows a macro-image of the patient""s eye to be transmitted as well along the optical axis, whereby X-Y polar array is arranged circumferentially about the directly viewed macro-image of the eye.